Electrochemical drilling system and process for improving electrical porosity of etched anode foil

ABSTRACT

A process for creating porous anode foil for use in an electrolytic capacitor of an implantable cardioverter defibrillator is provided. The process includes electrochemical drilling a plurality of etched metal foils in sequence one after the other in a bath containing electrochemical drilling (ECD) solution initially having a pH of less than 5. Alternatively, an etched foil sheet may be passed through the bath in a substantially continuous manner such that a portion of said etched foil sheet is in contact with the ECD solution is electrochemically drilled to generate pores. Electrochemical drilling is achieved when a current is passed to the foil or portion of the foil sheet in solution. ECD replenishment solution having a pH of less than about 5 is added from a feed reservoir to the bath at such a rate so as to maintain a pH in the ECD solution in the bath of less than about 5, and ECD solution in the bath is removed to a waste reservoir at the substantially the same rate as the addition of the ECD replenishment solution to the bath.

FIELD OF THE INVENTION

The present invention relates generally to implantable cardiac devices.More particularly, the invention relates to a system and process forcreating a porous anode foil for use in an electrolytic capacitor of animplantable cardioverter defibrillator (ICD), for example, a multipleanode stack configuration electrolytic capacitor.

BACKGROUND ART

Compact, high voltage capacitors are utilized as energy storagereservoirs in many applications, including implantable medical devices.These capacitors are required to have a high energy density since it isdesirable to minimize the overall size of the implanted device. This isparticularly true of an Implantable Cardioverter Defibrillator (ICD),also referred to as an implantable defibrillator, since the high voltagecapacitors used to deliver the defibrillation pulse can occupy as muchas one third of the ICD volume.

Implantable Cardioverter Defibrillators, such as those disclosed in U.S.Pat. No. 5,131,388, incorporated herein by reference, typically use twoelectrolytic capacitors in series to achieve the desired high voltagefor shock delivery. For example, an implantable cardioverterdefibrillator may utilize two 350 to 400 volt electrolytic capacitors inseries to achieve a voltage of 700 to 800 volts.

Electrolytic capacitors are used in ICDs because they have the mostnearly ideal properties in terms of size, reliability and ability towithstand relatively high voltage. Conventionally, such electrolyticcapacitors include an etched aluminum foil anode, an aluminum foil orfilm cathode, and an interposed kraft paper or fabric gauze separatorimpregnated with a solvent-based liquid electrolyte. While aluminum isthe preferred metal for the anode plates, other metals such as tantalum,magnesium, titanium, niobium, zirconium and zinc may be used. A typicalsolvent-based liquid electrolyte may be a mixture of a weak acid and asalt of a weak acid, preferably a salt of the weak acid employed, in apolyhydroxy alcohol solvent. The electrolytic or ion-producing componentof the electrolyte is the salt that is dissolved in the solvent. Theentire laminate is rolled up into the form of a substantiallycylindrical body, or wound roll, that is held together with adhesivetape and is encased, with the aid of suitable insulation, in an aluminumtube or canister. Connections to the anode and the cathode are made viatabs. Alternative flat constructions for aluminum electrolyticcapacitors are also known, comprising a planar, layered, stack structureof electrode materials with separators interposed therebetween, such asthose disclosed in the above-mentioned U.S. Pat. No. 5,131,388.

In ICDs, as in other applications where space is a critical designelement, it is desirable to use capacitors with the greatest possiblecapacitance per unit volume. Since the capacitance of an aluminumelectrolytic capacitor is provided by the anodes, a clear strategy forincreasing the energy density in the capacitor is to minimize the volumetaken up by paper and cathode and maximize the number of anodes. Amultiple anode stack configuration requires fewer cathodes and paperspacers than a single anode configuration and thus reduces the size ofthe device. A multiple anode stack consists of a number of unitsconsisting of a cathode, a paper spacer, two or more anodes, a paperspacer and a cathode, with neighboring units sharing the cathode betweenthem. Energy storage density can be increased by using a multiple anodestack configuration element; however, the drawback is that theequivalent series resistance, ESR, of the capacitor increases as theconduction path from cathode to anode becomes increasingly tortuous. Tocharge and discharge the inner anodes (furthest from the cathode) chargemust flow through the outer anodes. With typical anode foil, the paththrough an anode is quite tortuous and results in a high ESR for amultiple anode stack configuration. By keeping the ESR low, however, thecharge efficiency and DSR (delivered to stored energy ratio) of thecapacitor are maximized.

The conduction path from the cathode to the inner anodes may be madeless tortuous by providing pores in the outer anode foil. In thismanner, charge can flow directly through the outer anodes to the inneranodes. Thus, the use of porous anode foil can combat the increase inESR resulting from the use of a multiple anode stack configuration. U.S.Pat. No. 6,802,954 to Hemphill et al., incorporated herein by referencein its entirety, describes an electrochemical drilling process forcreating porous anode foil for use in multiple anode stack configurationelectrolytic capacitors which produces a pore structure that ismicroscopic in pore diameter and spacing, allowing for increased energydensity with a minimal increase in ESR of the capacitor. An etched foilis placed into an electrochemical drilling solution and a DC powersupply is used to electrochemically etch the foil in the electrochemicaldrilling solution such that pores on the order of a few microns diameterare produced through the foil. The electrochemical drilling processcreates large diameter “through” type tunnels, or pathways, in the foilthat increase the electrical porosity of the foil, thereby improvingcharge efficiency and DSR. A widening process may be employed to widentunnel diameter, maximizing surface area and reducing the taper oftunnels. A barrier layer oxide is formed on the anode foil, and thetunnel diameter should be large enough to leave a pore in the tunnelsdespite this oxide layer.

The electrochemical drilling process in accordance with U.S. Pat. No.6,802,954, however, utilizes an electrochemical drilling solution withan initially neutral pH that becomes slightly basic with a pH of aroundabout 9 to 11 shortly after starting processing of foils, causingaluminum dissolution in the solution to precipitate as aluminumhydroxide. This aluminum hydroxide solid should be filtered fromsolution if it is desired to process a plurality of foils; otherwise,the electrochemical drilling solution becomes less effective in creating“through” type tunnels that improve electrical porosity. Additionally,the aluminum hydroxide solid may build up on the process equipment,causing production downtime for routine cleaning (e.g., weekly) of theprocess equipment using a caustic solution. The electrochemical drillingsolution should be dumped routinely (e.g., daily) and replaced with newsolution free of solids.

What is needed, then, is a consistent and efficient method of creating aplurality of porous anode foil for use in capacitors that minimizes ESRwhile maintaining high capacitance.

BRIEF SUMMARY

An electrochemical drilling system and process for improving theelectrical porosity of etched foils are provided. In one embodiment, amethod for creating porous anode foils includes electrochemical drillinga plurality of etched metal foils in sequence one after the other, in abath containing an electrochemical drilling (ECD) solution initiallyhaving a pH of less than 5. An ECD replenishment solution having a pH ofless than about 5 is added to the bath at such a rate so as to maintaina pH in the ECD solution in the bath of less than about 5, and ECDsolution is removed from the bath at substantially the same rate as theaddition of the ECD replenishment solution to the bath.

In another method presented herein, an etched foil sheet is passedthrough the bath in a substantially continuous manner such that aportion of said etched foil sheet is in contact with the ECD solution. Acurrent is caused to flow through the ECD solution in the bath, so thatthe portion of the etched foil sheet in contact with the ECD solution iselectrochemically drilled to generate pores. ECD replenishment solutionis added to the bath at such a rate so as to maintain a pH in the ECDsolution in the bath of less than about 5. ECD solution is removed fromthe bath at substantially the same rate as the addition of the ECDreplenishment solution to the bath.

A system for creating a porous anode foil includes an electrochemicalbath containing an ECD solution initially having a pH of less than about5; a charge source connected to the bath to cause an electrochemicalreaction in the bath; a feed reservoir fluidly connected to theelectrochemical bath, the feed reservoir containing an ECD replenishmentsolution having a pH of less than about 5; a waste reservoir fluidlyconnected to the electrochemical bath for receiving ECD solution removedfrom the bath; and adding means for adding the ECD replenishmentsolution to the bath from the feed reservoir so as to maintain a pH inthe ECD solution in the bath of less than about 5. The bath may receivea plurality of etched metal foils in sequence one after the other or anetched foil sheet in a substantially continuous manner, with a portionof the etched foil sheet being in contact with the ECD solution.

The methods and system described herein provide the advantage of acontinuous ECD process, in which circulation of ECD solution to/from thebath achieves a substantially steady state concentration of dissolvedaluminum, which by maintenance of the solution pH, does not precipitateinto aluminum hydroxide. As a result, production downtime is reduced,since batch renewal of the ECD solution is avoided and caustic cleaningof the process equipment is less frequent.

Further embodiments, features, and advantages of the present system andmethod, as well as the structure and operation of the variousembodiments of the present system and method, are described in detailbelow with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate the methods and systems presentedherein for creating porous anode foil. Together with the detaileddescription, the drawings further serve to explain the principles of andto enable a person skilled in the relevant art(s) to make and use themethods and systems presented herein. In the drawings, like referencenumbers indicate identical or functionally similar elements. Further,the drawing in which an element first appears is typically indicated bythe leftmost digit(s) in the corresponding reference number.

FIG. 1 provides SEM photographs of a surface of a foil made with aconventional neutral pH electrochemical drilling solution (“NeutralECD”) and SEM photographs of a surface a foil made with a low pHelectrochemical drilling solution (“Low pH ECD”).

FIG. 2 provides an SEM photograph of a cross section of the “NeutralECD” foil shown in FIG. 1 and an SEM photograph of a cross section the“Low pH ECD” foil shown in FIG. 1.

FIG. 3 is a schematic of an system for electrochemically drilling anodefoil according to one embodiment of the present application.

FIG. 4 is a flowchart of a method for creating porous anode foilsaccording to an embodiment of the present application.

FIG. 5 is a flowchart of a method for maintaining a pH of anelectrochemical drilling solution in accordance with the method of FIG.4, according to an embodiment of the present application.

FIG. 6 is a graph of capacitance data from foils subject to an ECDprocess experiment of Example 4.

FIG. 7 is a graph of porosity data from foils subject to the ECD processexperiment of Example 4.

DETAILED DESCRIPTION 1. Overview

The following detailed description of methods and systems for creatingporous anode foil refers to the accompanying drawings that illustrateexemplary embodiments consistent with these methods and systems. Otherembodiments are possible, and modifications may be made to theembodiments within the spirit and scope of the methods and systemspresented herein. Therefore, the following detailed description is notmeant to limit the methods and systems described herein. Rather, thescope of these methods and systems is defined by the appended claims.

It would be apparent to one of skill in the art that the methods andsystems for creating porous anode foil, as described below, may beimplemented in many different embodiments without departing from thescope of the description below. Thus, the operation and behavior of themethods and systems will be described with the understanding thatmodifications and variations of the embodiments are possible, given thelevel of detail presented herein. It will be apparent to a personskilled in the relevant art that the methods and systems also beemployed to produce porous anode foil for use in a variety of devicesand applications in addition to use in an implantable cardioverterdefibrillator (ICD).

The present invention is directed to a method of creating porous anodefoil for use in electrolytic capacitors, particularly multiple anodestack configuration electrolytic capacitors, in order to reduce theequivalent series resistance (ESR) of such multiple anode stackconfigurations without sacrificing capacitance. According to a threestep etch process, such as that described in U.S. Pat. No. 6,802,954,incorporated herein by reference, a metal foil is etched to produce anenlargement of surface area; then the etched foil is then placed into aelectrochemical drilling solution and a DC power supply is used toelectrochemically etch the etched foil in the electrochemical drillingsolution to produce pores on the order of about 1 micron to about 1000microns in diameter through the foil, increasing its porosity; andfinally, the foil pores are widened and the foil is formed to theintended use voltage according to conventional widening and formingprocesses.

Initially, the anode metal foil is etched, according to a conventionaletch process, as known to those skilled in the relevant art, to producean enlargement of the surface area of the foil. The metal foil ispreferably aluminum foil, because of its ability to produce a sufficientquality oxide layer, its conductive properties, and its wide commercialavailability. However, other foils conventionally utilized inelectrolytic capacitors could also be used, including tantalum,magnesium, titanium, niobium, zirconium and zinc. Preferably, a 100 to125 micron thick, unetched, high purity (at least 99.98%) strip ofaluminum foil with high cubicity, wherein at least 80% of thecrystalline aluminum structure is oriented in a normal position (i.e., a(1,0,0) orientation) relative to the surface of the foil, is used. Suchfoils are well-known in the art and are readily available fromcommercial sources.

During the initial etching process, surface area of the foil isincreased by electrochemically removing portions of the foil to createetch tunnels, as disclosed in U.S. Pat. Nos. 4,474,657, 4,518,471,4,525,249 and 5,715,133. Since the capacitance of an electrolyticcapacitor increases with the surface area of its electrodes, increasingthe surface area of the anode foil results in increased capacitance perunit volume of the electrolytic capacitor. By electrolytically etchingan anode foil, an enlargement of a surface area of the foil will occur.Electrolytic capacitors which are manufactured with such etched foilscan obtain a given capacity with a smaller volume than an electrolyticcapacitor which utilizes a foil with an unetched surface.

Typically, an aluminum foil may be etched in a high temperature etchelectrolyte that is based on a halide and/or oxyhalide, such as achloride and/or oxychloride, and contains an oxidizer such as peroxide,persulfate, cerium sulfate or sodium periodate, at a pH of about 0.0 toabout 8.0, preferably a pH of about 1.0 to about 3.0. Other surface areaenhancing etch solutions can be used to produce similar results. In oneembodiment, the electrolyte etch solution consists of about 1.3% byweight NaCl and about 3.5% by weight NaClO₄. The electrolyte is heatedto a temperature of about 80° C. to about 100° C., with a preferredtemperature of about 85° C. The foil is placed in the etch electrolyteand etched at a current density of about 0.1 to about 0.3 amps/cm²,preferably about 0.15 amps/cm², and at an etch charge of about 5 toabout 50 Coulombs/cm² for a specific amount of time, preferably about 36Coulombs/cm² for about 4 minutes. In the preferred embodiment, the foilis etched to produce an enlargement of surface area of at least 20times.

In accordance with the present application, the etched foil is thenplaced into a bath containing an electrochemical drilling (ECD) solutionat a temperature from about 40° C. to about 100° C., typically betweenabout between about 91° C. and 98° C. The ECD solution of the presentapplication has a pH of less than about 5, and is maintained at a sub-5pH in accordance with the methods described in further detail below. Ifthe foil is left in the solution for more than 5 seconds withoutapplying a current to the foil, a hydration oxide layer will begin toform on the foil. The hydration oxide is not desirable, and should beremoved during the widening process before the surface area of thetunnels can be maximized. As a consequence of hydration removal, thefoil may have smaller tunnel diameters after widening, with reduced DSRand charge efficiency, and there may be significant variation inelectrical porosity between foils. Therefore, it is desirable to reducethe rate of hydration formation to decrease such variation of theelectrical porosity. One way to reduce the rate of hydration formationis to reduce the ECD solution temperature. Accordingly, in oneembodiment, the ECD solution is maintained at a reduced temperaturebetween about 80° C. and 95° C., preferably between about 80° C. and 90°C. Foils subject to the ECD process using an acidic ECD solution at atemperature of 85 and 90 degrees have also been found to havestatistically the same foil capacitance as foils subject to a neutralECD solution at a temperature of 95 degrees. Accordingly, an acidic ECDsolution in accordance with the present application can achieve adesirable electrical porosity and effectively drill foils without theassistance of a higher solution temperature, and the allowance of alower solution temperature reduces the rate of hydration formation andtherefore decreases the variation of the electrical porosity.

In one embodiment, the ECD solution is highly acidic, with a pH of equalto or less than about 3, and is maintained at or below a pH of 3 inaccordance with the methods described in further detail below. In oneembodiment, the pH is maintained within the pH range of 1 to 3, and inanother embodiment, the pH range is 0.5 to 3. In another embodiment, theECD solution is maintained at a pH at or below about 2. For an ECDsolution having a pH at or below about 3, the initial electrochemicaldrilling solution is made of about 1% to about 15% by weight sodiumchloride, preferably about 5% by weight, 0.1% by weight of an acid, andabout 10 to about 5000 PPM of a surface passivator, preferably about1000 PPM. The preferred surface passivator is sodium nitrate, but can beany alkali metal salt of nitrate, phosphoric acid or the alkali metalsalts of phosphate, and any of the soluble silicates, such as sodiumsilicate and potassium silicate, and the alkali metal salts of sulfate.The surface passivator helps to protect the foil surface and concentratethe current density from the power supply to smaller areas for tunnelformation. The concentrated current density creates more “through” typetunnels. The preferred acid is hydrochloric acid, but can also be otheracids such as phosphoric acid or nitric acid, for example.

As described in further detail below, an ECD replenishment solution isfed to the ECD bath during foil processing so as to maintain the desiredpH of the ECD solution in the bath. ECD solution is correspondingly bledfrom the bath as waste. As noted above, the etched foil is placed in thebath of ECD solution and current is applied to the etched foil so as toelectrochemically drill the etched foil. For example, as apparent to oneof skill in the relevant art(s), a charge source may be connected to thefoil (acting as an anode) and to cathode plate(s) to complete anelectronic circuit. During the ECD process, water is reduced at thecathode plates, causing increase in the pH of the ECD solution. FeedingECD replenishment solution maintains the pH, and bleeding ECD solutionas waste helps to remove aluminum in solution from dissolution of thefoil. Consequently, during the ECD process, an aluminum concentration inthe ECD solution achieves steady state, and this concentration can beoptimized for electrical porosity creation, which in turn should reducevariation in charge efficiency of the capacitor. Specifically, aluminumdissolution may be used as a measure of the electrochemical drilling. Ingeneral, the aluminum concentration in the ECD solution has an upperlimit past which the ECD process is not effectively improving theelectrical porosity of the foil. Therefore, the aluminum concentrationmay be maintained below this upper limit by adjusting the replenishmentsolution feed flow rate, and corresponding bleed flow rate, to and fromthe ECD bath. Since aluminum hydroxide is typically least soluble fromabove a pH of about 5 to about a pH of about 9, a maintained acidic pHof the ECD solution of less than about 5 prevents little to no formationof aluminum hydroxide solid in the solution and no filtering of thesolution is needed. Production downtime and costs should be reducedsince caustic cleaning is less frequent, if needed at all. Further,without aluminum hydroxide solid formation, variation in electricalporosity of multiple foils, or along a continuous foil sheet, isreduced, and there is no need to routinely replace the solution in thebath with new ECD solution.

To maintain the pH of the ECD solution at or below 3, it is preferredthat the ECD replenishment solution is substantially identical to theinitial ECD solution. For example, further to the above example, the ECDreplenishment solution should be made of 1000 ppm sodium nitrate, 5%sodium chloride, and 0.1% hydrochloric acid. It should be apparent toone of skill in the relevant art(s) that the pH of the initial ECDsolution and ECD replenishment solution may be adjusted to achieve adesired sub-neutral, or “low”, pH by adding more or less acid. Forexample, in one embodiment, 0.5% by weight hydrochloric acid is used inthe initial ECD solution and the ECD replenishment solution. Moreover,in one embodiment, if the initial ECD solution in the bath has a pHbelow that desired for electrochemical drilling, then dummy foils may berun through the ECD solution to raise the pH of the ECD solution priorto processing of foils. Thereafter, the ECD replenishment solution,having a lower pH than the solution now in the bath, may be added to thebath with little fluctuation in the pH of the solution in the bath.

An appropriate amount of electrochemical drilling produces a microscopicpore diameter and spacing which reduces ESR significantly. A DC powersupply is used to electrochemically drill the foil (or a portion of acontinuous foil sheet) at a constant current density and for a time ofabout five seconds to about 15 minutes, preferably between 10 sec and 3minutes in one embodiment, preferably about 45 sec in anotherembodiment, preferably about 1 min 45 sec (1:45) in another embodiment,preferably about 2 minutes in another embodiment, and at a temperaturefrom about 40° C. to about 100° C., preferably between about 80° C. and90° C. The applied current density should be from about 0.1 to about 1.0amp/cm², preferably about 0.2 amps/cm². The etch charge is varied fromabout 1 Q/cm² to about 50 Q/cm², preferably 18 Q/cm², to produce thedesired number and size of electrochemically drilled holes.

FIGS. 1 and 2 provide a comparison of an aluminum anode foilelectrochemically drilled using a conventional neutral pHelectrochemical drilling solution (i.e., solution excludes HCl; foillabeled as “Neutral ECD”) and an anode foil electrochemically drilledusing a low, acidic pH electrochemical drilling solution (i.e., solutionincludes HCl; foil labeled as “Low pH ECD”). FIG. 1 shows SEMphotographs (providing two levels of magnification) of a surface of the“Neutral ECD” anode foil and a surface of the “Low pH ECD” anode foil.FIG. 2 shows SEM photographs of cross-sectional views of theserespective foils, showing tunnels prior to any widening process. Asshown in FIG. 1, the “Neutral ECD” foil has more surface erosion andaluminum chunk removal near areas of large tunnel formation. Areas oflarge tunnel formation provide high electrical porosity sites on thefoil surface. The “Low pH ECD” foil also has high electrical porositysites with high tunnel initiation, but having a more even surface,without significant surface erosion. The circled dark-colored portionsin FIG. 1 indicate pores produced by the electrochemical drillingprocess. As shown in FIG. 2, the “Low pH ECD” foil has more tunnelsspread out, though less tunnels go all the way through the foil and aregenerally not as long as the tunnels formed in the “Neutral ECD” foil.Notwithstanding, both foils have areas of long tunnels that would createa desirable electrical porosity pore. It is desired for the resultingpore size to be about 1 micron to about 1000 microns in diameter withpore to pore spacing of about 1 micron to about 100,000 microns, morepreferably about 3 microns in diameter with spacing of about 15 micronsbetween pore centers.

The etched foil can be masked so that only small areas of the etchedfoil are exposed to the electrochemical drilling solution. In oneembodiment, the etched foil is held between two masks with a grid ofopenings which expose the masked foil. The masked foil is then placedinto the electrochemical drill solution and a DC power supply is used,as discussed above, to further electrochemically etch the exposed areasof the foil. The electrochemical drill is allowed to continue until theappropriate pore size has been created.

The spatial arrangement of unmasked areas may be chosen from a number ofirregular patterns, such as disclosed in U.S. Pat. No. 6,802,954. Apattern that allows the reduction of ESR, the maintenance of strengthand the maintenance of capacitance is preferred for the mask. Thepattern is configured in such a way that the enhanced area does notcreate large scale strength defects such as perforation holes, divots,chunk removal and the like. The exposed area can be as little as about10% of the total foil area to as much as about 95% of the total foilarea, and is preferably about 30% to about 70% of the total foil area.In one embodiment, the mask is held tight around the whole foil and theedges of the foil are blocked from the electrochemical drillingsolution. Preferably, a thin mask having small tapered holes of lessthan 1 mm is used, to concentrate the applied current density and toallow bubbles formed during the electrochemical drilling process toescape more easily.

Next, the foil may be rinsed in an overflow deionized (DI) water bathfor a time of about 1 to about 10 minutes, preferably about 1.5 minutes.

The foil pores are then widened in a chloride or nitrate containingelectrolyte solution known to those skilled in the art, such as thatdisclosed in U.S. Pat. Nos. 3,779,877 and 4,525,249. Then the foil isdipped into a deionized water bath at a temperature of about 80° C. toabout 100° C., preferably about 95° C., to form a hydrate layer on thefoil surface.

Next, a barrier oxide layer can optionally be electrochemically formedonto one or both surfaces of the metal foil, sufficiently thick tosupport the intended use voltage, by placing the foil into a formingsolution. Useful forming solutions include, but are not restricted to, asolution based on azelaic acid, sebacic acid, suberic acid, adipic acid,dodecanedioic acid, citric acid or other related organic acids andsalts. Preferably, a citric acid solution is employed. This step ispreferably conducted at a temperature of about 80° C. to about 100° C.,preferably about 85° C., at a current density of about 1 mA/cm² to about40 mA/cm², preferably about 16 mA/cm². A formation voltage of about 50to about 800 Volts, preferably about 445 V, can be applied to the foilto form the barrier oxide layer. The barrier oxide layer provides a highresistance to current passing between the electrolyte and the metalfoils, also referred to as the leakage current. A high leakage currentcan result in the poor performance and reliability of an electrolyticcapacitor. In particular, a high leakage current results in greateramount of charge leaking out of the capacitor once it has been charged.

A heat treatment of 500° C.±20° C. may be applied to the foil followingformation for about 1 to about 10 minutes, preferably about 4 minutes.The foil is then returned to the forming solution and allowed to soakwith no applied potential for about 1 to about 10 minutes, preferablyabout 2 minutes. A second formation in the same electrolytic formingsolution at high temperature is performed at a potential of about 435Volts.

Next, the foils may be dipped in a suitable low concentration,oxide-dissolving acid solution, including but not restricted to,phosphoric acid, formic acid, acetic acid, citric acid, oxalic acid, andacids of the halides, preferably phosphoric acid, at an acidconcentration of about 1% to about 10% by weight, preferably aconcentration of about 2% by weight, at a temperature of about 60° C. toabout 90° C., preferably about 70° C., for a time of about 1 to about 10minutes, preferably about 4 minutes.

Finally, the foils are reformed at a voltage of about 435 Volts in asuitable forming solution, as discussed above, at a high temperature,preferably about 80° C. to about 100° C., more preferably about 85° C.

2. System and Methods for Sub-Neutral ECD

FIG. 3 is a schematic of a system 300 for an ECD process according toone embodiment of the present invention. The system may be employed toelectrochemically drill a plurality of etched foils in sequence oneafter the other or drill a continuous etched foil sheet in asubstantially continuous manner.

System 300 includes an ECD bath 320, a feed reservoir 310, a wastereservoir 330, a metering pump 340, and a charge source 380 for passinga current through the bath, to cause an electrochemical reaction. In theembodiment shown, charge source 380 electrically connects anode foil 370to cathode plates 390 placed adjacent opposing faces of foil 370. ECDsolution 322 is contained in bath 320; ECD replenishment solution 312 iscontained in feed reservoir 310; and discharged solution 332 bled frombath 320 is contained in waste reservoir 330. System 300 may furtherinclude a separate circulating pump 350 for churning the ECD solution inbath through inlet and outlet pipes 352 a and 352 b, respectively, toensure mixing of ECD solution 322 in bath 320 with ECD replenishmentsolution 312 fed from reservoir 310. A pipe 342 a fluidly connects feedreservoir 310 with metering pump 340, and a supply pipe 342 b fluidlyconnects pump 340 with bath 320. A discharge pipe 362 fluidly connectsbath 320 with waste reservoir 330. Discharge pipe 362 is provided with asolenoid valve 360 for controlling the amount of ECD solution 322discharged from bath 320. A pH meter measuring the pH of ECD solution322 in bath 320 is connected to solenoid valve 360, so that when the pHrises to a predetermined control limit, solenoid valve opens and ECDsolution 322 is bled from bath 320. A solution level detector 324 sensesif the solution bath 320 is below a predetermined control limit. Outputfrom level detector 324 may be used to control pump 340, whereby whenthe solution level is below the predetermined control limit, pump 340begins to feed replenishment solution 312 from feed reservoir 310. Bleedflow rates (through pipe 362) and feed flow rates (through pump 340) maybe used to set control limits on the time periods pump 340 is operatedand/or solenoid valve 360 is opened, thereby controlling the volume ofsolution fed or bled to/from bath 320.

In one embodiment, the initial ECD solution 322 in bath 320 and ECDreplenishment solution 312 have a sub-neutral pH, with the respective pHof the solutions each being less than about 5. In another embodiment,the pH initial ECD solution 322 in bath 320 and ECD replenishmentsolution 312 are each less than or equal to 3, each being made of about0.1% by weight of an acid, preferably hydrochloric acid. An exemplaryfeed flow rates to maintain the pH of ECD solution 322 in bath 320 isabout 250 g/min per 50 liters of ECD solution in the bath.

It should be apparent that other embodiments of arrangements and controlmethods for system 300 than that depicted in FIG. 3 may be constructed,without departing from the spirit and scope of the ECD system andprocess described herein. For example, pH meter 326 may be connected topump 340 so as to activate pump 340 when the pH of solution 322 in bath320 reaches a predetermined pH, and level detector 326 may measure whensolution 322 is above a predetermined level so as to open solenoid valve360. Further, instead of solenoid valve 360, pipe 362 may be joined toan overflow port on bath 320, whereby solution 322 is discharged frombath 320 when it reaches above a predetermined level (due to feed ofreplenishment solution 312).

Bath 320 may be rectangular tank with an open top, or otherwise havingopening(s) on one or more of its walls for entry and/or exit of theetched foils. Bath 320 may be configured to receive a plurality ofetched foil sequentially. Further, bath may receive a set of one or moreetched foils at one time, followed by additional sets of one or morefoils processed in sequence. Alternatively, bath 320 may be configuredto receive an etched foil sheet in a substantially continuous manner.For example, foil wound on a roller may be unrolled as a continuous foilsheet and passed through the electrochemical bath in a substantiallycontinuous manner. The foil may then be rerolled downstream of bath 320subsequent to the ECD process. As should be apparent, the unrolling androlling of the foil sheet, and feeding of the foil sheet through the ECDbath, may be automated. For example, motors may be provided on rollersover which the foil sheet is wound. Additional motorized and/orfree-wheeling rollers may be used to support the continuous foil sheetalong its movement through bath 320. Commonly-owned U.S. applicationSer. No. 10/745,016 to Stocker et al., incorporated by reference hereinin its entirety, describes etching a foil sheet in a substantiallycontinuous manner using a plurality of rollers (e.g., rollers 600 a-fshown in FIG. 6 of that patent application). A similar configuration ofrollers as described in U.S. application Ser. No. 10/745,016 may beemployed for passing an etched foil sheet through bath 320 betweencathodes 390 in a substantially continuous manner.

The phrase “substantially continuous” is used herein to mean the foil ispassed through the electrochemical bath in a manner such that the foilis electrochemically drilled substantially uniformly along its length.For example, the substantially continuous manner includes, but is notlimited to, passing the foil through the bath at a constant speed. Thesubstantially continuous manner further includes passing the foilthrough the bath at an intermittent speed, which means the foil ispassed through the electrochemical bath for a period of time and thefoil is stopped or at rest for a period of time. The period of time foreach cycle can vary, and the speeds at which the foil is passed throughthe bath may further be dependent on the dimension of the tank. Forexample, if it is desired to subject a portion of the foil in the ECDbath for 1.5 to 2.5 minutes with a given applied current (or othermeasure of the coulombs applied to the foil), the speed for passing thefoil through in a continuous manner may be based on the length of thetank though which the foil passes. The cycle of passing and stopping maybe repeated, in a continuous manner, until the desired amount of foilsheet has passed through the electrochemical bath for the desired totalamount of time. Accordingly, passing the foil through the bath in asubstantially continuous manner may expose the foil to the ECD bath fora similar length of time as the processing of individual foils insequence.

FIGS. 4 and 5 illustrate steps of a process 400 for creating porousanode foil, in accordance with the present application. As with system300, process 400 may be employed to electrochemically drill a pluralityof etched foils in sequence one after the other or drill a continuousetched foil sheet in a substantially continuous manner. Accordingly, instep 410 of process 400, either a plurality of etched foils are passedsequentially through a bath of ECD solution or an etched foil sheet iscontinuously passed through the bath. As noted above, the bath has a pHbelow about 5, so as to be sub-neutral, and at least slightly acidic. Instep 420, a current flows through the solution and is thereby applied tothe foil or portion thereof in contact with the ECD solution, wherebythe foil or portion thereof is electrochemically drilled to generatepores. In step 430, ECD replenishment solution having a pH of less thanabout 5 is added in order to maintain the pH of the bath during foilprocessing. ECD solution in the bath is removed in step 440. To maintainthe levels of bath and ensure steady state aluminum concentration, it ispreferred that ECD solution is removed from the bath at substantiallythe same rate as the addition of the ECD replenishment solution to thebath.

In one embodiment, the pH of the solution in the bath is highly acidic,with a pH of equal to or less than about 3, and is maintained at this pHin accordance with process 400. The ECD replenishment solution maytherefore have a pH of equal to or less than about 3 so as to maintainthe bath within this pH range. The ECD replenishment solution and theECD solution initially contained in the bath comprise about 0.1% byweight of an acid. As noted above, preferably, the acid is hydrochloricacid, and the ECD replenishment solution and the ECD solution initiallycontained in the bath include about 1000 PPM of a surface passivator andabout 5% by weight sodium chloride. In this instance, to maintain the pHof equal to or less than about 3 in the bath, with two 252 cm² etchedmetal foils exposed to the bath at the same time and a total run timefor the two foils of about two minutes, exemplary feed and bleed flowrates are about 250 g/min per 50 liters of ECD solution in the bath,with a total of thirty 252 cm² etched metal foils undergoing theelectrochemical drilling step in about 30 minutes. The ECD replenishmentsolution and the solution bled from the bath may be continuously orintermittently added/bled in accordance with such flow rates. Forexample, in one embodiment, about 1250 g of ECD replenishment solutionis added intermittently every 5 minutes, and in another embodiment, abatch of about 2500 g of ECD replenishment solution is added every 10minutes. In another embodiment, every 30 minutes between about 1% to 5%of the total volume of ECD solution in the bath is bled and replenishedwith ECD replenishment solution.

Feeding and bleeding of ECD solution may be made automatic byimplementing a control system to monitor the pH of the bath and controlflow feed and bleed flow rates. FIG. 5 illustrates steps 532 and 534 formaintaining the pH in the bath by the addition of ECD replenishmentsolution, in accordance with step 430 of process 400. In step 532, thepH of the ECD solution in the bath is monitored, and in step 534, theamount of ECD replenishment solution added to the bath is controlledbased on pH monitored in step 532. For example, as described above withreference to system 300 of FIG. 3, a pH meter may be used to measure thepH of ECD solution in bath 320, and its output may be used to open asolenoid valve to bleed ECD solution from the bath when the pH rises toa predetermined control limit (e.g., a pH of 3). Control settings willdictate the amount solution to bleed (and replenish) so that the desiredpH is maintained.

Porous anode foil produced using an ECD process with a sub-neutral ECDsolution, according to the methods described herein, when used in amultiple anode stack configuration, will exhibit the same or better foilcapacitance and the same or reduced ESR compared to a substantiallyneutral ECD process using a neutral to basic ECD solution. The porousfoil is suitable for commercial use in an electrolytic capacitor with amultiple anode stack or wound roll configuration. Thus, the presentapplication is further directed to an electrolytic capacitor havingporous anode foil provided by etch, ECD, and widening processesdescribed herein.

Anode foils that are processed according to the methods described abovecan be utilized for a variety of applications that require a highcapacitance anode foil. For example, as discussed above, anode foils arewidely utilized in electrolytic capacitors, such as those described inU.S. Pat. No. 5,131,388 and U.S. Pat. No. 5,584,890, incorporated hereinby reference. Electrolytic capacitors, which are manufactured with anodefoils etched according to the present invention, can obtain a givencapacity with a smaller volume than currently available electrolyticcapacitors and, therefore, can be very compact in size.

Electrolytic capacitors manufactured with porous anode foils created inaccordance with the methods described herein can be utilized in ICDs,such as those described in U.S. Pat. No. 5,522,851, incorporated byreference herein in its entirety, such that the increased capacitanceper unit volume of the electrolytic capacitor allows for a reduction inthe size of the ICD. Thus, the present application is further directedto an ICD utilizing an electrolytic capacitor having porous anode foilprovided by the process described herein.

Having now generally described the invention, the same will be morereadily understood through reference to the following examples which areprovided by way of illustration, and are not intended to be limiting ofthe present invention.

3. Examples

The following Examples 1-3 show ECD process experiments in which usingan “acid ECD solution” that includes 0.1% by weight of acid, as areplacement for a “neutral ECD solution” excluding such acid, gives thesame or better foil capacitance, foil capacitance standard deviation,porosity, punch yield, and capacitor electrical requirements. In each ofthe examples, the acid ECD solution is 5% sodium chloride, 1000 ppmsodium nitrate, and varying hydrochloric acid. The neutral ECD solutionis 5% sodium chloride and 500 ppm sodium nitrate. The results for theacid ECD foils from the experiments of Examples 1-3 are summarized belowin Table 1. Exemplary process control limits described herein areapplicable to both acidic and neutral ECD solutions.

Example 1 200 Etched Foils

Rolls of etched aluminum foils were used. After an ECD station wascaustic cleaned using the caustic cleaning procedure, the stock neutralECD solution was heated to 95° C. Once at 95° C., 25 grams of sodiumnitrate to the solution was added to the ECD tank. Additionally, 370grams of 5.0% HCl was added to the solution in the ECD tank, to make thestock solution in the tank an acid ECD solution. The pH was measuredafter running 2 sets of dummy foils. The pH was about 2.0. The ECD timewas set at 1 minute and 24 seconds at 63.0 amps. Every 10 foils, 44grams of the 5.0% HCl solution was added to the ECD solution. If the pHincreased above 2.7, a second 44 grams of 5.0% HCl solution was added.

At the end of the 200 foils, 1672 grams of 5.0% HCl was added to thetank not counting the initial 370 g add. Therefore, the estimated HClneeded per foil was 0.42 g/foil. The filter was inspected for solidbuild up. The amount of solids on the filter was very minimal. Thesolution after 200 foils was slightly gray.

The second 200 foils were processed as normal using a neutral ECDsolution, with no addition of HCl.

The average foil capacitance of the acidic ECD foil was 328.4 μF (per252 cm²) and the average foil capacitance of the normal ECD foil was330.7 μF. The average foil capacitance was 1.30 μF/cm² (above a 1.22μF/cm² test plan minimum). The standard deviation of the foilcapacitance of the acidic ECD foil was 2.4%. The average ESR of 4 sheetsof the acidic ECD foil tested at the beginning and after every 50 foilswas 28.4 ohms and desirably below a 35 ohm upper process control limit.By keeping the ESR low, the charge efficiency and DSR (delivered tostored energy ratio) of the capacitor are maximized. In a finishedcapacitor, it is desirable to keep the DSR as close to 1 as possible.Therefore, a lower process control limit (e.g., 35 ohms) for ESR of eachanode foil permits multiple stacked anodes to be used in a capacitorwhile maximizing DSR. Moreover, a multiple anode stack configurationpermits reduction in the overall size of the implanted device.

The punch yield of the acidic ECD foil was 17.5 anodes out of 28 anodes,which is above a 16 anode minimum desired. Typically, a foil sheet maydivided to produce 28 anodes. Punch yield refers to the quality of thefoil, reflecting the number of useable anodes produced, with nodefective anodes produced when the punch yield is 28 anodes, and thedifference from 28 being the number of defective anodes (e.g., crackedfoils).

Example 2 200 Etched Foils

After the ECD station was caustic cleaned using a caustic cleaningprocedure, the stock neutral ECD solution was heated to 95° C. Once at95° C., 25 grams of sodium nitrate was added to the ECD tank.Additionally, 1000 grams of 5.0% HCl was added to the solution in theECD tank, to make the stock solution in the tank an acid ECD solution.The pH was measured after running 6 sets of dummy foils. The pH wasabout 0.56. The ECD time was set at 1 minute and 45 seconds at 50.4amps. Every 10 foils, 44 grams of the 5.0% HCl solution was added to theECD solution. If the pH increased above 2.7, a second 44 grams of 5.0%HCl solution was added. At the end of the 200 foils, 1518 grams of 5.0%HCl was added to the tank not counting the initial 1000 g add.Therefore, the estimated HCl needed per foil was 0.38 g/foil. The filterwas inspected for solid build up. The amount of solids on the filter wasvery minimal. The solution after 200 foils was slightly gray.

The second 200 foils were processed as normal using the stock neutralECD solution, with no addition of HCl.

The average foil capacitance of the acid ECD foil was 314.8 μF (per 252cm²) and the average foil capacitance of the normal ECD foil was 313.0μF. The average foil capacitance was 1.25 μF/cm² and above the 1.22μF/cm² test plan minimum. The standard deviation of the foil capacitanceof the acid ECD foil was 2.6%. The average ESR of 4 sheets tested at thebeginning and after every 50 foils was 28.2 ohms and desirably below a35 ohm upper process control limit. The punch yield was 20.6 anodes outof 28 anodes, which is above a 16 anode minimum desired.

The foil drilled using the acidic ECD solution of Examples 1 and 2 werecombined to make capacitors. The average delivered energy of thecapacitors was 18.9 J with a DSR of 0.896. Therefore, the foil assembledinto capacitors gave the appropriate delivered energy (above nominal of18.75 J) and the DSR was high indicating high porosity.

Example 3 400 Etched Foils

The ECD station was not caustic cleaned using the caustic cleaningprocedure before the experiment. Therefore, the experiment shows a worsecase scenario of excess aluminum in the system.

Once at 95° C., 25 grams of sodium nitrate to the solution was added tothe ECD tank. Additionally, 1000 grams of 5.0% HCl was added to thesolution in the ECD tank. The pH was measured after running 6 sets ofdummy foils. The pH was about 0.79.

The ECD time was set at 1 minute and 45 seconds at 50.4 amps.

Every 10 foils, 44 grams of the 5.0% HCl solution was added to the ECDsolution. If the pH increases above 2.7, a second 44 grams of 5.0% HClsolution was added.

At the end of the 400 foils, 2904 grams of 5.0% HCl was added to thetank not counting the initial 1000 g add. Therefore, the estimated HClneeded per foil was 0.36 g/foil.

The filter was inspected for solid build up. The amount of solids on thefilter was very minimal. The solution after 400 foils was slightly gray.The cathodes during the process did have some solid build up, but didnot seem to be a problem and was easily removed.

The average foil capacitance of the acid ECD foil was 321.6 μF (per 252cm²). The average foil capacitance was 1.28 μF/cm² and above the 1.22μF/cm² test plan minimum. The standard deviation of the foil capacitanceof the acid ECD foil was 3.1% and below the 3.5% maximum. The averageESR of 8 sheets tested at random was 21.4 ohms and desirably below a 35ohm upper process control limit. The punch yield was 17.2 anodes out of28 anodes, which is above a 16 anode minimum desired.

The foil was used to make capacitors. The average delivered energy ofthe capacitors was 18.75 J with a DSR of 0.891. Therefore, the foilassembled into capacitors gave the appropriate delivered energy (abovenominal of 18.75 J) and the DSR was high indicating high porosity.

TABLE 1 # etched Average foils for Foil ESR Ω Acid Capacitance (per #Punch ECD μF/cm² Cap. std dev. sheets) Yield Example 1 200 1.30 2.4%28.4 (4) 17.5 Example 2 200 1.25 2.6% 28.2 (4) 20.6 Example 3 400 1.283.1% 21.4 (8) 17.2

Example 4

Twelve (12) foils were subjected to an ECD process to determine ifporosity and capacitance met specification at an ECD solution pH between0.5 and 1.0. The HCl concentration in the ECD solution was changed toadd 6100 ml of 37% HCl per 700 liters of final ECD mix to maintain pHbetween 0.5 and 3.0. The ECD temperature specification was 85-95° C.with a target of 91° C.

The 12 foils processed at a pH between 0.5 and 1.0 met porosity andcapacitance specification. The measured capacitance (μF/252 cm²)relative to upper and lower process control specifications is shown inFIG. 6, and the measured ESR (Ω) relative to upper and lower processcontrol specifications is provided in FIG. 7.

4. Conclusion

It is to be appreciated that the Detailed Description section, and notthe Summary and Abstract sections, is intended to be used to interpretthe claims. The Summary and Abstract sections may set forth one or morebut not all exemplary embodiments of the present system and method ascontemplated by the inventors, and thus, are not intended to limit thepresent method and system and the appended claims in any way.

Moreover, while various embodiments of the present system and methodhave been described above, it should be understood that they have beenpresented by way of example, and not limitation. It will be apparent topersons skilled in the relevant art(s) that various changes in form anddetail can be made therein without departing from the spirit and scopeof the present system and method. Thus, the present system and methodshould not be limited by any of the above described exemplaryembodiments, but should be defined only in accordance with the followingclaims and their equivalents.

In addition, it should be understood that the figures, which highlightthe functionality and advantages of the present system and method, arepresented for example purposes only. Moreover, the steps indicated inthe exemplary system(s) and method(s) described above may in some casesbe performed in a different order than the order described, and somesteps may be added, modified, or removed, without departing from thespirit and scope of the present system and method.

Further, the purpose of the foregoing Abstract is to enable the U.S.Patent and Trademark Office and the public generally, and especially thescientists, engineers and practitioners in the art who are not familiarwith patent or legal terms or phraseology, to determine quickly from acursory inspection the nature and essence of the technical disclosure ofthe application. The Abstract is not intended to be limiting as to thescope of the present system and method in any way.

What is claimed is:
 1. A method for creating a plurality of porous anodefoils, comprising: electrochemical drilling a plurality of etched metalfoils in sequence one after the other, in a bath containingelectrochemical drilling (ECD) solution initially having comprising 10to 5000 PPM of a surface passivator and about 0.1% by weight of an acid,wherein the ECD solution has a pH within a range of 0.5 to 3, andfurther wherein said electrochemical drilling step includes: adding ECDreplenishment solution having comprising 10 to 5000 PPM of the surfacepassivator and about 0.1% by weight of an acid, wherein the ECDreplenishment solution has a pH within a range of 0.5 to 3, to the bathat such a rate so as to maintain the pH in the ECD solution; anddischarging solution from the bath at substantially the same rate as theECD replenishment solution is added to the bath.
 2. The method of claim1, further comprising widening pores in each metal foil after eachetched metal foil has been electrochemically drilled.
 3. The method ofclaim 1, wherein the adding step further comprises: monitoring the pH ofthe ECD solution in the bath; and controlling the amount of ECDreplenishment solution added to the bath based on the monitored pH,wherein the ECD replenishment solution is added when the pH of the ECDsolution in the bath nears about
 3. 4. The method of claim 1, whereinthe acid is selected from the group consisting of hydrochloric acid,phosphoric acid, and nitric acid.
 5. The method of claim 4, wherein theacid is hydrochloric acid.
 6. The method of claim 5, wherein the ECDreplenishment solution and the ECD solution initially contained in thebath further comprise about 1000 PPM of the surface passivator and about5% by weight sodium chloride.
 7. The method of claim 6, wherein atemperature of the ECD solution in the bath is between about 80° C. and90° C.
 8. The method of claim 1 wherein the rate of adding the ECDreplenishment solution is about 250 g/min per 50 liters of ECD solutionin the bath, wherein about two etched metal foils undergo theelectrochemical drilling step in about two minutes.
 9. The method ofclaim 8, wherein a batch of about 1250 g of ECD replenishment solutionis added every 5 minutes or a batch of about 2500 g of ECD replenishmentsolution is added every 10 minutes.
 10. A method for creating a porousanode foil, comprising: passing an etched foil sheet through a bathcontaining electrochemical drilling (ECD) solution initially havingcomprising 10 to 5000 PPM of a surface passivator and about 0.1% byweight of an acid, wherein the ECD solution has a pH within a range of0.5 to 3, in a substantially continuous manner such that a portion ofsaid etched foil sheet is in contact with the ECD solution; causing acurrent to flow through the ECD solution in the bath, so that saidportion of said etched foil sheet in contact with the ECD solution iselectrochemically drilled to generate pores; adding ECD replenishmentsolution having comprising 10 to 5000 PPM of the passivator and about0.1% by weight of an acid, wherein the ECD replenishment solution has apH within a range of 0.5 to 3; and discharging solution from the bath atsubstantially the same rate as the ECD replenishment solution is addedto the bath, wherein the ECD replenishment solution is added to the bathat such a rate so as to maintain the pH in the ECD solution in the bath.11. The method of claim 10, further comprising widening pores in saidetched foil sheet after said foil sheet has been electrochemicallydrilled.
 12. The method of claim 10, wherein the ECD replenishmentsolution and the ECD solution initially contained in the bath compriseabout 1000 PPM of the surface passivator and about 5% by weight sodiumchloride.